At first, we shall discuss some properties that are typical of cellular telecommunication systems. The discussion will concentrate on digital cellular systems, such as GSM (Global System for Mobile Communications) and TETRA (Terrestrial Trunked Radio). Digitalization reduces the expenses of operators and users and facilitates the building of interfaces to other digital data transfer systems. Compared to an analogue, trunked radio system, the operation of a digital radio system is more versatile, because it has added properties, which the analogue system lacks. In addition, data transfer is more reliable and the frequency band is utilized more efficiently.
An example of digital cellular telecommunication systems is TETRA, which is a new, European standard for trunked radio systems. TETRA is an example of a PMR (Professional Mobile Radio) network, which is used by organizations, such as the police or the fire brigade, in order to get certain properties to their own communication network. The most important properties of a system according to the TETRA standard include firstly, encryption of messages to prevent outsiders from accessing the messages, secondly, the priority of the message, which guarantees that the most important message is heard and interrupts the others, and thirdly, group operations, by means of which a certain group communicates easily and reliably.
ETSI (European Telecommunications Standards Institute) has developed the TETRA standard together with leading producers, system operators and users. TETRA is a cellular system, in which a certain centre manages a relatively large number of base stations. Both actual payload data and control data, or signalling, are transferred between the base stations and mobile equipment. The user does not hear the signalling, and thus need not pay attention to it. The users, or actually the mobile equipment used by them, may constitute groups, in which each piece of equipment belonging to a certain group can hear all the equipment in the group. The way how connections of the point-to-multipoint type required by group communications are formed on the network side is not essential with regard to the present invention.
The processing of audio data in a piece of mobile TETRA equipment will be discussed briefly. A corresponding discussion is also more generally valid for a typical piece of mobile communication equipment in a cellular network. A Coder-Decoder, or CODEC, codes (or encodes) the audio data to be transmitted and decodes the received coded audio data. In the general manner, coding consists of source coding (speech encoding) and channel coding, and decoding consists of channel decoding and source decoding (speech decoding). In TETRA, the source coding method is parametric coding, in which the waveform of the sound is not coded directly, but certain parameters are calculated from it, and on the basis of that the receiving device synthesizes the original sound and produces a copy of it. Thus speech decoding can also be called speech synthetization in TETRA.
TETRA is a Time Division Multiple Access (TDMA) network, in which each transmission frequency has four independent transmission channels and the difference between transmission frequencies is 25 kHz. The TDMA transmission frame is ca. 56.67 ms long and it consists of four time slots of even length. In a single time slot, the transmitting device transmits a digital burst, which represents a voice sequence of 60 ms. Consecutive TDMA frames form a superframe structure, which is repeated at intervals of 1.02 seconds and in which 17 TDMA frames are followed by a signalling frame as the 18th frame.
The data rate in TETRA may vary from 2.4 kBit/s to 28.8 kBit/s. The lowest rate is in use when one time slot is used for data transfer and thus very reliable operation is required. The highest rate is achieved when the protection protocol is not used at all and when all four time slots of the TDMA frame can be used for data transfer.
One of the properties of cellular systems is the above mentioned possibility to use group connections. The users of equipment in a group connection may be members of more than one group. If there are several groups, speech groups have different priorities. This ensures that the user always hears the message with the highest priority, i.e. the one classified as the most important. When a message with a higher priority is received, the device silences messages with lower priority and reproduces the one with the highest priority. The operational area of the speech group may be either the whole network or part of it, depending on the user's need. The messages of the speech group are not heard outside a certain area.
Problems have occurred in group connections when two communication devices of the same group are close to each other and the first of them transmits speech or other audio data to the group. A corresponding problem also occurs in a connection between two devices, if the devices are sufficiently close to each other even for a moment. As an example, we shall consider a situation in which the user has two devices: a communication device installed in a car and a mobile communication device which can always be carried by the user. It is assumed that both communication devices are in the group mode and they are connected to the same user group. Now the user wants to send a message to the other members of the group. When the user speaks to the group through the communication device installed in the car, the transmitted sound message is also received almost simultaneously in the user's mobile communication device. The mobile communication device receives the transmitted sound message after the period of time needed for the sound message to proceed in the network. Then the mobile communication device reproduces the sound, whereupon the communication device installed in the car catches the sound message again and transmits it back to the network. This creates a situation in which the sound circulates.
The customary echo cancelling alone does not remove this problem, because in it the purpose is to prevent the message reproduced by the loudspeaker of the device from proceeding to the network again via the microphone of the same device. Acoustic echo cancelling functions so that when the microphone of the device catches the sound just reproduced by the loudspeaker, it can be attenuated and it is thus practically not re-transmitted to the network. In the present audio circulation problem, the re-circulating sound comes from the loudspeaker of another device, and information about it is not obtained by the customary echo cancelling means in the other device, which now sends the sound to circulate.